When you open the plunger on the end of the pump which attaches to the faucet, it sucks water from the tank. When you close it, it sends water to the tank. The green thing that attaches to the faucet has a thing on the end of it where you open and close the nozzel of the pump (plunger). Works slick. There's also a lever by the gravel vac used to cut off the suction or the adding of water to the tank.

As for the dechlorinator, just add enough to the tank to treat the whole tank and fill it back up. No need to do anything special. Once I switch the flow of the water, I put in enough Prime to treat 55G, which is about a capful.

And one more thing, when filling back up, before you close the plunger, you can adjust the temp of the faucet, then just close the plunger and fill back up. I've been able to do 50%-75% PWC's and change the temp in the tank no more than .5 degrees. It's a wonderful tool.

The python is a God-send for anyone who has multiple tanks and/or tanks over 10 gal.

Basically, the T-shaped Python fitting that attaches to the faucet is a water aspirator.
Inside, the running tap water is forced through a cone-shaped constriction. This causes a drop in pressure and creates suction in the side piece that the gravel vac is attached to. The running tap water and siphoned tank water exit through the bottom of the fitting.

To refill the tank, you twist the bottom of the Python fitting to seal it shut. Now, the fitting ceases to be a T-shaped water aspirator, and becomes a 90o fitting. Running tap water flows through the side fitting, through the gravel vac, and into the tank.

As Lonewolfblue said, you adjust the water temp by feel at the tap, and dechlorinator is added as the tank is refilling.

The Python is an amazing peice of equipment...I've used one for 10 years now and couldn't imagine trying to do tank maintance without one. I don't know anyone who's serious about their tank that doesn't use one.

If you are on a budget, just an eager DIY-er, you can buy the parts and assemble one of these yourself for less than Python charges.

Go to the local waterbed outlet and look at their acessories. They will have a venturi just like Python's but half as much and not green. It should come with faucet adapters, but I like brass instead of plastic, so I would go to the local hardware and get brass adapters. While there, get a CHEAP 1/2" or 5/8" garden hose (50 footers are common for less than $10). The female end will thread onto the venturi. You can cut off the male end for a tank-friendly simple drain-n-fill. If you want to attach other accessories (vacuum, filter, etc.) you can also keep the male fitting or substitute in a plastic one. Hundreds of fittings out there. Sky's the limit.

I've got a Python with a snap-to-connect faucet coupler intended for a dishwasher setup (about $15, all brass, from the local hardware). Let's me couple and uncouple the python from the faucet w/o having to worry about cross-threading the aerator or finding the strap wrench for "too-tight" situations...

__________________
Just tipping into planted aquaria after twenty years of fish-keeping.

Listen to Lunewolfblue in all things! And as far as adjusting the temp before adding the water back in, if you have a removeable thermometer, it is very easy to do. Just note your tank temp., then pop your thermometer out, hold it under your tap, and adjust the temperature of the tap water to get it as close as possible to the tank temp. Once you've got it, reverse your water flow, pour in your water conditioner, and fill 'er up!